1. Field of the Invention
The present invention is in the field of telecommunications and, in particular, in the field of communication systems using protocol layers for processing information to be transmitted and/or for processing received information.
2. Description of Prior Art
In mobile wireless communication environment it is a challenging task to provide reliable high-quality services due to a dynamic behavior of a communication link, for example of a wireless link. Therefore, system designers have to cope with a non-predictable variation of transmission quality resulting from time-varying resource availability, fading errors, outages or handover. For wireless networks beyond the third generation systems (B3G), this dynamic behavior will negatively be affected since B3G systems are expected to span across heterogeneous wireless access network technologies with different transmission characteristics. However, the next generation wireless networks are expected to provide reliable and transparent services to the customers so that a seamless use of the network's diversity can be achieved.
Service and application provisioning in B3G does not only have to regard network's density but also application diversity as new business models which are expected to allow third party providers to offer their application on top of the operators' service platforms making use of advanced open interfaces. In order to take dynamically changing application requirements into account, which may result e.g. from varying user preferences or varying user context, the operators will need an ability to dynamically change the systems parameters in order to react to the varying requirements.
Usually, conventional communication systems apply a plurality of communication layers arranged to a protocol stack for information processing. FIG. 9 shows a protocol stack comprising a plurality of hierarchically arranged communication layers. The prior art protocol stack shown in FIG. 9 is disclosed in Andrew Tannenbaum, Computer Networks, 4th Edition, Francis Hall, 2003.
The protocol stack comprises a physical layer 901, a data link layer 903 arranged above the physical layer 901, a network layer 905 arranged on top of the data link layer 903, a transport layer 907 arranged at top of the network layer 905, and an application layer 911 arranged at top of the transport layer 907.
Generally speaking, the application layer is operative for managing the information to be transmitted. For example, the information comprises a media data stream, for example a video data stream, as information to be transmitted through a communication channel. Alternatively, the information may comprise a multimedia data stream consisting of video and audio information, to be transmitted through the communication channel. Furthermore, the application may comprise an electronic mail, etc. In other words, the application layer is operative for transforming the application to be transmitted into a transmittable information stream.
The application layer 911 directly communicates with the transport layer 907 being operative for providing a transport service, so that the information can be transmitted to a destination sink in dependency of the physical network used for communication. For example, the transport layer appends a transport protocol data unit (TPDU) to the information data stream in order to preserve a peer-to-peer communication which is common in all communication networks. Peer-to-peer communication means that for example the transport layer 907 communicates directly with another transport layer implemented in a destination network.
The transport layer 907 communicates directly with the network layer 905 being operative for processing an information frame provided by the transport layer 907, so that an end-to-end communication, i.e. communication between two computer entities, is possible.
The network layer 905 provides a network layer frame to a link layer comprising the data link layer 903 and the physical layer 901, wherein the data link layer 903 and the physical layer 901 may comprise a plurality of sub-layers, for example a medium access control sub-layer.
The link layer is operative for managing the transmission of the information represented by bits through the communication channel. For example, the data link layer 903 is operative for applying a forward error correction encoding (FEC) or forward error detection encoding, for re-transmission of erroneous data frames (packets) and, for example, for confirming of a correct reception of each frame by sending an acknowledgement frame. Furthermore, the data link layer 903 may be operative for scheduling the frames to be transmitted in, for example, a multi-user scenario. Scheduling means, that a frame is transmitted at a predetermined time slot (transmission time frame).
The data link layer 903 directly communicates with the physical layer 901 being operative for further encoding the streams provided by the data link layer 903 by, for example, performing a modulation using a modulation scheme modulating a carrier according to the information to be transmitted.
The embodiment of the protocol stack shown in FIG. 9 corresponds to a TCP/IP reference model described in the above-referenced document (TCP=transmission control protocol, IP=internet protocol). For the sake of convenience it is to be noted, that the protocol stack shown in FIG. 9 also corresponds to the OSI reference model (OSI=open system interconnect) with exception of two layers, namely a session layer and a presentation layer arranged between the application layer 901 and the transport layer 907.
The internet protocol stack as depicted in FIG. 9 is expected to be used as a basic platform for B3G systems and applications. However, in order to achieve a good transmission quality, within a varying transmission environment, an efficient use of the available network resources is necessary in order to adapt the communication system or the application to come up, for example, to varying transmission characteristics and application requirements. For example, in case of a frequency-selective communication channel, a suitable encoding of the data bit stream to be transmitted is necessary, so that a predetermined bit error probability, i.e. 10−6, is not increased. To do so, the physical layer may be, for example, operative, to adapt the modulation scheme to the current channel characteristic. Accordingly, a system adaptation can be performed on all protocol layers of the protocol stack by adapting the respective parameters determining an operation mode of a respective communication layer.
Conventionally, the optimization of the system for a specific application, for example a video stream, is performed in a vertical manner in a system carrying only one service in a non-layered scenario, e.g. in a POTS system (POTS=Plain Old Telephony Service).
In layered communication systems, such as wireless internet, traditionally, certain layers are independently optimized for an expected worst case scenario (worst condition), which results in an inefficient use of the available communication resources, for example in available bandwidth, an achievable data rate associated with a certain bit error probability etc.
However, in existing systems, the intra-layer adaptation is performed without considering inter-layer dependencies. In P. A. Chou, and Z. Miao, “Rate-Distortion Optimization Streaming of Packetized Media”, Technical Report MSR-TR-2001-35, Microsoft Research, Microsoft Corporation, February 2001, a communication system is disclosed, where a media frame scheduling is performed by the application layer, wherein only in inter-dependency of the media frames transporting video and audio information is taken into account. In M. Kalman, E. Steinbach, and B. Girod, “R-D Optimized Media Streaming Enhanced with Adaptive Media Playout”, International Conference on Multimedia and Expo, ICME 2002, Lausanne, August 2002, an adaptive media playout scheme is described, where the playout speed of audio data (for example voice) and video data is varied as a function of channel conditions. In S. Saha, M. Jamtgaard, J. Villasenor, “Bringing the wireless Internet to mobile devices”, Computer, vol. 34, issue 6, pp. 54-58, June 2001, an adaptive middle layer is described, that applies transcoding of media data in order to adapt the currently used coding scheme to varying channel conditions. In H. Imura et al., “TCP over Second (2.5G) and Third (3G) Generation Wireless Networks”, IETF RFC 3481, February 2003, a wireless TCP protocol stack is described, that distinguishes between packet losses due to a network congestion and losses due to erasures on a wireless link. In F. H. Fitzek, and M. Reisslein, “A prefetching protocol for continuous media streaming in wireless environments”, IEEE Journal on Selected Areas in Communications, vol. 19, no. 10, pp. 2015-2028, October 2001, a data link layer re-transmission is described, where a delay constraint is taken into account. The known differentiated services approached (DIFFSERV) is based on an established priority among media packets, so that more important media packets are preferably scheduled. Additionally, adaptive modulation and encoding on the physical layer is known, as for example described in the IEEE 802.11a standard.
However, the above indicated prior art approaches suffer from the fact, that only one layer is optimized with respect to fulfilling an optimization goal. For example, in order to improve a transmission quality, the physical layer may be operative to adaptively adjust the transmission power depending on a current channel condition, for example a current channel attenuation. In other words, the above indicated prior art approaches rely on an optimization of only one parameter set determining an operation mode of the respective communication layer.
In order to more efficiently exploit the resources, an adaptation of two layers can be performed. In K. Stuhlmüller, N. Färber, and B. Girod, “Analysis of video transmission over lossy channels”, IEEE Journal on Selected Area in Communication, vol. 18, no. 6, pp. 1012-1032, June 2000, and T. Fingscheidt, T. Hindelang, R. V. Cox, N. Seshadri, “Joint Source-Channel (De)Coding for Mobile Communications”, IEEE Transactions on Communications, Vol. 50, No. 2, pp. 200-212, February 2002, a source and channel coding scheme is described. The adaptation scheme is based on an adaptation of a source rate and code rate depending on the channel conditions in terms of transmission quality. To be more specific, an analytic formula is disclosed enabling a calculating of a source rate and of a channel rate.
In W. Yuan, K. Nahrstedt, S. Adve, D. Jones, R. Kravets: Design and Evaluation of a Cross-Layer Adaptation Framework for Mobile Multimedia Systems, to appear in SPIE/ACM Multimedia Computing and Networking Conference (MMCN) 2003, an optimization of power control and transmission data rate is disclosed. In S. Toumpis, A. Goldsmith: Performance, Optimization, and Cross-Layer Design of Media Access Protocols for Wireless Ad Hoc Networks, IEEE International Conference on Communications (ICC), 2003 a medium access control (MAC) layer and physical layer optimization for ad hoc networks are described.
However, the prior art concepts applying cross-layer design for optimization purposes suffer from a disadvantage, that, within the communication system, only a certain optimization approach is considered for the intra-layer adaptation. Moreover, the prior art approaches do not consider inter-layer dependencies which results in an ineffective exploitation of the available resources.
A further disadvantage of the prior art approaches is that the disclosed optimization schemes are not flexible. Since the prior art approaches indicated above only consider one or two certain parameters for optimization, for example power control and transmission data rate, further optimization scenarios are not considered in order to fully exploit the available communication resources.